The EPR spectra of the spin labels attached to loops L11 and L12 of kinesin were resolved into slow (rotational correlation time, tau = 10-45 ns) and fast (tau = 2 ns) components. The fraction of the slow component increased considerably when kinesin was complexed with a microtubule (MT). On MT binding and in the presence of nucleotides ADP and AMPPNP, the spin labels on L11, particularly at A252C and L249C, significantly decreased the fraction of the slow component. Moreover, dipolar EPR detected a wide distribution in distance range, 1-2 nm between the two spin labels attached to T242C/A252C or A247C/A252C; this distribution was slightly narrower in the presence of MTs than in their absence. These results suggested that the L11 residues undergo conformational transition on the binding of nucleotides and MT, while these residues remained to fluctuate over a nanometer range. (C) 2007 Elsevier Inc. All rights reserved.